Something remarkable happened on St Nicholas's Day, December 6, 1273. Saint Thomas Aquinas, the theological genius of the medieval period - a man who had set down millions of words on matters in nature and logic, in metaphysics and morality - abruptly put down his pen. He did not write again. When begged to continue, he replied he could not because "all I have written seems like straw".

The reference to straw did not mean his output suddenly seemed worthless to him. Rather, straw was a metaphor for basics: like the crude material used for building a house, his words now seemed like rough stuff. His goal had been to understand God. He had made many verbal attempts on the summit. But while his efforts had produced wonderful reflections he had reached the point where he was able to appreciate the most profound truth - the peak lies beyond. God is unknown. His silence was not a rejection but the culmination of his work.

Today, it seems, many believers forget that the first and last thing that can and should be said about God is that God is unknown. Consider the kind of evangelical religiosity in which worship ceases to be an encounter with the mystery of God and becomes instead a feelgood experience, a time for sharing with a deity who is so well known that you can even ask him with help parking the car at the supermarket. As Kierkegaard quipped, if faith would turn water into wine, this turns wine into water.

Or take the speech of the Pope that offended so many Muslims. He argued that Christian theological reasoning reveals the hidden God, because reason - in the second person of the Trinity, the logos - is part of God. To my mind, he forgot the Socratic principle that reason is the key to wisdom not because it shares all understanding, but because it exposes understanding's limits.

And I think this is precisely what Aquinas's silence was about too: it manifests what might be called the agnostic spirit implicit in the religious quest. It is easy to sideline how radically agnostic Aquinas was about God, even as a man of faith. He argued that theology is different from science. In science you know what you are studying (say in physics, the physical world); and its nature and scope (in physics, the universe). However, with theology, you know that it is about God. But the nature and scope of God are entirely unclear.

So, the way all good theology proceeds is by saying what God is not: God is immortal, invisible, unknown. Even the existence of God cannot be asserted, Aquinas thought; and neither God's nonexistence. Such agnosticism is disconcerting. But discarding it turns theology from the pursuit of God into the construction of a deity - that is idolatry.

The agnostic spirit typically irritates atheists too. They say it puts God above reason and so beyond proof. Well, yes. We are talking about God. Take Aquinas "proofs" of God, one of which is that God is the "unmoved mover", the source of the movement of all things. The mistake is to presume Aquinas thinks this shows God's existence. He does not. The aim of the proofs is to show how incapable reason is of grasping God. That is its value, to throw us beyond anything that can be said, to God.

Why do this kind of theology? Many philosophers and theologians have testified that the truths of existence are beyond expression. So words about God must be spoken, but ultimately only so a point is reached at which they stop. The final stroke of Aquinas's genius, then, was not crafted by his quill, but was conveyed by his silence. Mark Vernon is the author of Science, Religion and the Meaning of Life (Palgrave Macmillan, 2006)